TW201736264A - A process for production of ammonia from inert-free synthesis gas in multiple reaction systems - Google Patents

Abstract

In a process for the production of ammonia in at least two reaction systems, in which ammonia is produced from a portion of the synthesis gas in each of the systems with a part-stream being withdrawn, the make-up gas is essentially inert-free, the downstream system is at the same pressure or at a higher pressure than the upstream system and the make-up gas is sent once through a make-up gas (MUG) converter unit, the residual synthesis gas coming from the MUG converter unit is optionally pressurized to a higher pressure before being sent to an inert-free synthesis loop. This way, an economically attractive production of ammonia is feasible with synthesis gases not containing inerts.

Description

Method for preparing ammonia from a non-inert synthesis gas in a multiple reaction system

This invention relates to a process for the preparation of ammonia from a non-inert synthesis gas in at least two reaction systems. More specifically, ammonia is produced from a non-inert synthesis gas in a multi-pressure process according to the reaction N ₂ +3 H ₂ -> 2 NH ₃ (1) in at least two reaction systems.

According to the reaction in the high pressure synthesis loop (1) ammonia is produced from the synthesis gas by a catalytic reaction between hydrogen and nitrogen. In addition to hydrogen and nitrogen, the ammonia synthesis gas contains components which are generally inert to the reaction (1), such as methane and noble gases, which hinder the conversion rate of the reaction (1) and will be referred to as "inert group" hereinafter. Or "only" is called "inert matter." This type of process typically operates in such a manner that the make-up gas is first compressed to a high pressure in several stages and then the compressed make-up gas is fed to a circuit that encompasses one or more catalyst-filled reactors to produce ammonia. It is known in the art to feed a high pressure circuit by replenishing a synthesis gas which is obtained from a steam reforming of a hydrocarbon feed such as natural gas in a suitable molar ratio (i.e., 3 to 1). ) consists of H ₂ and N ₂ .

In order to avoid enrichment in the loop of the inert component contained in the extracted ammonia and soluble only at very low concentrations, a partial stream of the gas circulating in the loop is continuously withdrawn as the flushing gas. The residual ammonia is removed from the flushing gas by scrubbing, by using membrane technology or cryogenic separation Remove and recover hydrogen and nitrogen (if present). The residual inert component, if present, is withdrawn, such as methane, argon, helium, and residual nitrogen. The recycle gas is added to the make-up gas prior to compressing the make-up gas, and thus the make-up gas is used again. The energy balance is unfavorable for extracting a large amount of flushing gas system from the circuit, since this will cause a significant drop in the pressure of the bulk gas, which must subsequently undergo secondary compression and cause a lot of expense.

It is believed by those skilled in the art that the enrichment of the inert substance from the initial value of 1 vol% to 2 vol% in the make-up gas to 10 vol% or even 20 vol% cannot be avoided in the recycle gas even if there is a gas participating in the reaction. The partial pressure (which is only critical to the equilibrium of the reaction to the state of the equilibrium of the reaction) is significantly lower than the inevitable disadvantage of its partial pressure in a completely non-inert synthesis gas loop, which is equivalent to this Inert substance concentration is associated. This is the volume of catalyst used and the reactor containing it must be significantly larger than the volume of catalyst required for the absence of inert components in the synthesis gas loop and the reactor.

The enrichment of the inert material in the circuit that can withstand the disadvantages described above, compared to the original concentration level in the supplemental gas, indicates a technical anomaly due to the following: in the presence of a smaller amount of flushing gas and thus Operating costs (especially compression-related operating costs) are reduced in the case of high concentrations of inert components, while resource costs are attributed to the large amount of required media or the need to use more expensive catalysts (eg, based on Catalyst) increases. This technical anomaly cannot be solved using current state-of-the-art technology, and this is why it is forcing experts in the field to find a compromise and establish an optimal cost balance relative to high operating expenses and capital costs.

The synthesis occurring in the reactor produces a product gas from the synthesis gas. This product gas consists essentially of the unreacted portion of the feed gas, the ammonia formed, and the inert components. Ammonia is gaseous at the outlet of the reactor, but it must be condensed so that it can be separated from the product gas and also acts as Liquid ammonia is drawn from the circuit. Since the dew point of ammonia depends on its partial pressure and its temperature, the advantage of product condensation is to provide a higher synthesis pressure and a higher ammonia concentration on the one hand and a lower temperature on the other hand. High ammonia concentrations can be obtained by using a large touch media product at low inert matter concentrations. The high synthesis pressure results in a correspondingly higher cost of energy required to compress the synthesis gas, and a lower cooling temperature requires an appropriate cooling device to be installed in the recycle gas line.

The above considerations disclose that those skilled in the art will generally tend to maintain the process synthesis pressure between 150 and 280 bar. Since the volume of the conventional magnet catalyst will grow disproportionately in the case of a decrease in the synthesis pressure, and since this also applies to the structural requirements of the reactor, the process described in the art uses a highly reactive catalyst. Therefore, a magnet catalyst doped with cobalt has been used in a large amount. Terpene catalysts have also been used, but these catalysts are more expensive due to the precious metal content.

The lower the synthesis pressure, the lower the amount of heat that can be dissipated by the use of water or air cooling, and therefore the portion of the heat to be removed by the refrigeration will therefore increase. As in standard practice, if a cooling circuit with a compressor system is required to account for refrigeration, this leads to another technical anomaly. Although the compression expenditure of the synthesis loop decreases as the synthesis pressure decreases, the compression expenditure of the cooling circuit increases because more refrigeration is required to extract the ammonia produced in the synthesis loop. The portion of the condensed ammonia prior to refrigeration is increased in the low pressure process because the very low inert component concentration is set by means of the high flow rate of the flushing gas stream. The problem with enrichment of the inert component will occur as in the high pressure synthesis process, and the lower inert concentration increases the product concentration and thus increases the dew point. Therefore, those skilled in the art must also find a compromise in this situation and establish an optimal cost balance with respect to high operating expenses and investment costs.

In the most conventional ammonia plant, natural gas is treated in the primary and secondary reformers to produce hydrogen, and after the residual heat has recovered from the reformed gas stream, the reformed gas stream is then subjected to a shift conversion for additional hydrogen production. . In a further step, the acid gas removal, and in a methanator downstream of the residual carbon monoxide (CO) and carbon dioxide (CO ₂₎ is converted into methane. The resulting raw synthesis gas stream is then passed to a synthesis loop for the production of ammonia, wherein nitrogen is typically supplied from the process air fed to the secondary reformer.

Typically, the ammonia plant will use a stoichiometric amount of process air in the secondary reformer to maintain a 3 to 1 hydrogen to nitrogen molar ratio in the methanator off-gas (the original synthesis gas), which is typically to the ammonia synthesis loop. Supplemental gas.

Commercial scale preparation of ammonia has been carried out in large single reaction systems for many years. The single reaction system is the result of the high cost associated with the circuit operating at high pressures and the high cost of the compression process, both of which experience a significant drop as the flow rate increases. Thus, there have been some technical biases for decades that state that the preparation of economically attractive ammonia is only possible in a single reaction system and only by a syngas system containing inert materials.

One of the first attempts to use more than one reaction system is disclosed in DD 225 029 A3, and DD 225 029 A3 describes two high pressure synthesis units that are successively configured and operate at the same pressure level. The first synthesis unit is a supplemental gas system and the second synthesis unit is a conventional loop system. The synthesis gas used must contain an inert material, and during the process, the concentration of the inert material is relatively high, more specifically 13 vol% to 18 vol% in the recycle gas.

It is known from US 7.070.750 B2 that ammonia can be produced from synthesis gas in a multi-pressure process in which the synthesis of ammonia takes place in at least two lined-up synthesis systems. According to this U.S. patent, ammonia is prepared from one portion of the synthesis gas in each system, wherein a portion of the stream is withdrawn, and the respective downstream synthesis systems operate at a higher pressure than the respective upstream synthesis systems. In this regard, "higher pressure" means a differential pressure that exceeds the pressure loss in the synthesis system. Each synthesis system can borrow Separated from the next downstream synthesis system by at least one compression stage.

In the method described in US 7.070.750 B2, all of the at least two synthesis systems operate as supplemental gas systems, except for the last synthesis system operating as a recycle loop system.

The process disclosed in US 7.070.750 B2 produces ammonia, compound-to-reaction (from the synthesis gas containing reactants H ₂ and N ₂ and compounds such as methane and noble gases) according to the reaction (1) mentioned above ( 1) Inert, which hinders the rate of conversion of reaction (1). In order to avoid enrichment in the loop of the inert compound, a partial flow of the gas circulating in the loop is continuously extracted as a flushing gas. It is recognized in US 7.070.750 B2 that inert compounds constitute a problem because their concentration in the recycle gas increases from an initial value of 1 vol% to 2 vol% in the make-up gas to as much as 10 vol% or even 20 vol%, resulting in The partial pressure of the gas participating in the reaction is significantly lower than its partial pressure in the non-inert synthesis gas loop. This disadvantage is typically compensated for by using a larger touch media product and thus using a larger reactor or alternatively by using a more efficient (and also more expensive) catalyst, such as a germanium based catalyst. According to US 7.070.750 B2, the multi-pressure process described therein produces satisfactory results, although inert compounds are permanently present in the synthesis gas.

The present invention is based on the idea that ammonia can be produced from a non-inert synthesis gas in at least two reaction systems according to the above reaction (1), wherein the downstream system is at or at a pressure higher than the upstream system. The synthesis gas or make-up gas system is from a nitrogen wash unit (NWU) or other cleaning unit in which all inert compounds have been removed to the ppm level. This means that the ammonia synthesis loop is non-inert for all practical purposes and therefore does not require a flushing system.

In the present invention, the terms "synthesis gas" and "supplement gas" are used interchangeably.

Accordingly, the present invention is directed to a method of producing ammonia in at least two reaction systems, the at least two reaction systems comprising an alignment synthesis system comprising a first system and a final system, wherein - in the at least two systems Each of which produces ammonia from a portion of the ammonia synthesis gas, wherein a portion of the stream is withdrawn, - the make-up gas is substantially non-inert, - the downstream system is at or at one of the same pressure as the upstream system Under pressure, and - the synthesis gas or make-up gas is delivered at a time via a make-up gas (MUG) converter unit, and wherein the residual synthesis gas from the MUG converter unit is transferred to a non-inert The synthesis loop is optimally pressurized to a higher pressure before the condition.

The make-up gas system preferably comes from a nitrogen wash unit (NWU).

The first system in the synthesis system line operates as a single stream reactor system. All of the at least two synthesis systems can operate as a single stream reactor system with the exception of the last synthesis system. The last synthesis system operates as a recirculation loop system.

In the synthesis system line, each synthesis system is combined with the next downstream by the compression phase. Separated into systems.

Since the circuit is non-inert, no flushing system is required. The supplemental gas is extremely reactive because there is no inert material.

The advantage of the MUG converter unit being below the pressure level of the main circuit is that it is much easier to control the exothermic reaction (1) and to obtain a reasonable reactor size for the MUG converter.

The invention is further explained with reference to the drawings in which the nitrogen purge unit NWU delivers a make-up gas having an inert content of a certain amount (which is virtually zero).

The ammonia synthesis gas can be pressurized after leaving the NWU, which is done in a first compressor stage/unit (CSU I) and then converted via a make-up gas (MUG) The unit transmits it once. This MUG converter unit (indicated as a dotted box in the figure) consists of the MUG converter itself (MUG conv.) together with cooling and condensing (c&c) components.

Pressurizing the residual synthesis gas from the MUG converter unit to a non-inert synthesis loop where the liquid ammonia is prepared, in a second compressor stage/unit (CSU II) high pressure.

The invention will be further illustrated by the following examples.

Example

Table 1 shows a key figure for comparing a 3000 MTPD ammonia plant based on a non-inert synthesis loop with a 3000 MTPD ammonia plant based on a non-inert make-up gas and a supplemental gas converter unit placed at three different pressure levels. It is shown that it is possible to prepare at least 20% ammonia in the MUG unit.

Considering that circular flow can be used as an indicator of the size of a synthetic loop device, The MUG unit reduces the size of the synthesis loop by at least 15%. This reduction in the size of the synthesis loop represents a possible capital expenditure savings, but more importantly, it provides the possibility of constructing a higher capacity ammonia plant in the form of a new device or in the form of a capacity increase of the device.

It should be noted that the number of preparation and circulation flows can be further optimized.

Claims (7)

A method of producing ammonia in at least two reaction systems, wherein ammonia is produced from a portion of the ammonia synthesis gas in each of the at least two systems, wherein a portion of the stream is withdrawn, the supplemental gas is substantially non-inert, the downstream system At or under the same pressure as the upstream system, and delivering the synthesis gas or supplemental gas at a time via a make-up gas (MUG) converter unit, and where The residual synthesis gas of the MUG converter unit is optimally pressurized to a higher pressure before being transferred to the non-inert synthesis circuit. The method of claim 1, wherein the make-up gas system is from a nitrogen washing unit (NWU). The method of claim 1, wherein the first synthesis system operates as a single flow reactor system. The method of claim 1, wherein all of the at least two synthesis systems operate as a single stream reactor system except for the last synthesis system. The method of claim 1, wherein the last synthesis system operates as a recycle loop system. The method of claim 1, wherein each synthesis system is separated from the next downstream synthesis system by one or more compression stages. The method of claim 1, wherein the downstream system is in phase with the upstream system Under the same pressure.